Lubricating oil can reduce friction between members and is used to assist the operation of many mechanical constituents. For example, the lubricating oil that is employed in an automobile gearbox (automobile gearbox oil), in addition to lubrication or cooling of gears, is employed in order to perform friction control or hydraulic operation during the engagement of a wet clutch. However, increase in bubbles in the lubricating oil causes problems such as impeding the cooling action, fluctuation of hydraulic pressure and acceleration of oxidative deterioration, and may eventually lead to failure of the hardware of the gearbox or the like. Lubricating oil therefore usually contains a defoaming agent. Also, in circumstances in which the viscosity of the lubricating oil is lowered with a view to saving fuel costs, the lowered viscosity results in increased foaming properties, so careful attention must be paid to anti-foaming measures.
In general as a defoaming agent for lubricating oil, rather than fluorine-based defoaming agents, to be described, ordinary silicone-based defoaming agents (such as polydimethylsiloxane) are employed: in particular, defoaming agents are frequently employed whose dynamic viscosity at 25° C. is in the range of from 50,000 to 1,500,000 mm2/s. Outside this range, defoaming properties at high temperature become unsatisfactory. A preferable range is from 80,000 to 1,200,000 mm2/s. A polydimethylsiloxane constituent may be employed on its own or as a combination of two or more types thereof. Regarding the blending amount, the polydimethylsiloxane constituent is preferably blended with the base oil in a ratio of from 1 to 50 weight ppm, based on the total weight of the composition. If the blending amount is less than 1 weight ppm, a defoaming effect may not be produced; if the blending amount exceeds 50 weight ppm, a beneficial effect matching the blending amount may not be achieved. More preferably, the blending amount is in the range of from 3 to 30 weight ppm.
Fluorine-based defoaming agents have therefore been proposed as defoaming agents having a high defoaming capability. For example, fluorine-modified silicones have applicability to low-viscosity lubricating oils of high solubility, since they are more insoluble with respect to the lubricating oil than the conventionally employed silicone-based defoaming agents. Furthermore, fluorine-modified silicones also have high defoaming capability at high temperature. However, fluorine-modified silicones have a large specific gravity, and so tend to separate out within the container prior to filling. Thus, even though fluorine-based defoaming agents exist that have a high latent capability as defoaming agents, since some of these are problematic in regard to storage stability, the present situation is that fluorine-based defoaming agents are not effectively utilised as defoaming agents for lubricating oil.
Japanese Patent Publication 60000086B proposes a different technique from the technique of mixing a fluorine-based defoaming agent with the lubricating oil. Specifically, Japanese Patent Publication 60000086B discloses a defoaming agent having excellent defoaming properties including a fluorine-containing compound in which a C3 to C20 fluorinated aliphatic group and a high aliphatic group of carbon number 10 or more are included in the same molecule. Also, as the method of use of this defoaming agent, apart from the technique of mixing with the lubricating oil, the technique of applying the defoaming agent to the rim of the container is proposed. The benefit of this latter technique is explained to be that any bubbles rising to the top are dissipated when they come into contact with the surface where the defoaming agent has been applied (page 5 block 6 to page 4 block 7).
Thus, although a defoaming effect can be obtained for a short period with the fluorine-based defoaming agent described in Japanese Patent Publication 60000086B with the technique described (technique of applying the defoaming agent to the rim of the container), there is the problem that a defoaming effect cannot be achieved in this way over a long period. Also, however high the defoaming effect, if fluid is employed, as has been the case up to the present, over a long period, this will flow downwards due to its weight, so it is thought that in general a prolonged effect will be better sustained by using a gel-form composition of high NLGI grade. Furthermore, if a defoaming agent in the form of a paste employing solid microparticles of silica as indicated in Japanese Patent Publication 60000086B is used, the silica constitutes a solid foreign body that causes abrasive wear, which has an adverse effect on lubrication performance.
A further problem was that, since the conventional defoaming agent was mixed with the lubricating oil beforehand, the amount of the defoaming constituent was diminished during use owing to capture by the filter. Furthermore, in an automatic gearbox (AT, CVT), the lubricating oil is circulated within the container by using a hydraulic pump and there is a possibility that foaming may occur during this process: such foaming results in poor lubrication, which may have consequences such as loss of the ability to control the device.
Accordingly, an object of the present invention is to provide technology for defoaming lubricating oil whereby a high defoaming capability can be maintained over a long period, employing a defoaming agent of a gel-form composition constituting a solid lubricant.